Recirculating system having partial bypass around the center wall



Oct. 26, 1965 R. w. EGGLESTONE 3,213,835

RECIRCULATING SYSTEM HAVING PARTIAL BYPASS AROUND THE CENTER WALL Filed July 27. 1961 3 Sheets-Sheet l FIGJ sEr GOVERNOR) Y SET - gay 30 74 "map-7,25" w 4 55% 1 COA/TAQLZ-R Lk F. I G 7 4 E2 42 Q T & R; q 400 200 z/vrmqmy aim/L4.

INVENTOR ROBERT W- EGGLESTONE ATTORNEY Oct. 26, 1965 R. w. EGGLESTONE 3,213,835

RECIRCULATING SYSTEM HAVING PARTIAL BYPASS AROUND THE CENTER WALL INVEN'I'OR ROBERT W- EGGI ES'TONE ATTORNEY Oct. 26, 1965 R. w. EGGLESTONE 3,213,835

RECIRCULATING SYSTEM HAVING PARTIAL BYPASS AROUND THE CENTER WALL 2&0

INVEN'I'OR BY hwy aim ATTORNEY ROBERT W- EGGLES'TONE United States Patent 3,213,835 RECIRCULATING SYSTEM HAVING PARTIAL BYPASS AROUND THE CENTER WALL Robert W. Egglestone, West Hartford, Conn, assignor to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed July 27, 1961, Ser. No. 127,386 18 Claims. (Cl. 1224tl6) This invention relates to recirculating systems and heat indicators in vapor generators and particularly to a recirculating system for a generator having sections of differing flow areas arranged in series.

An object of this invention is an improved recirculating system for a vapor generator having series arranged sections of different flow areas.

Another object is mechanism increasing the efficiency of the recirculating system.

Still another object is mechanism for reducing the loss in head in the recirculating and through-flow systems.

A still further object is mechanism retaining a selected velocity of fluid flow in a section of a once through-flow vapor generator of a predetermined flow area and reducing the velocity of fluid flow in a series arranged section of smaller flow area.

A still further object is mechanism giving a rapid and accurate indication of heat release in the furnace or heat picked up by furnace walls.

Other features and advantages will be apparent from the following specification and claims and from the accompanying drawings which illustrate embodiments of the invention and in which:

FIG. 1 is a diagrammatic representation of a vapor generator of the forced flow once-through type of operation in the supercritical presure range, and having sections of fluid heating tubes of different flow areas arranged in series and having, in accordance with the invention a recirculation circuit superimposed upon the tubes, the flow therethrough being apportioned in accordance with the flow areas and automatically controlled by a constant speed, uncontrolled, recirculating pump floating on the once-through flow; the lateral and rear gas passages have been moved to one side and the economizer has been omitted for clarity;

FIG. 2 is a schematic view of structure of FIG. 1 illustrating the invention and showing the recirculating pump in a recirculating line having a bypass around the center wall and connected in parallel with the throughfiow line fed by the feed pump;

FIG. 3 is a schematic view of a modification showing the recirculating pump at the water wall outlet, in the through-flow line in series with the feed pump, and a recircuating line having a bypass around the center wall;

FIG. 4 is a schematic view of a modification showing the recirculating pump at the inlet of the center wall, in the through-flow line in series with the feed pump, and a recirculating path including a bypass around the center wall;

FIG. 5 is a schematic view of a modification showing the recirculating pump between the center and water walls, in the through-flow line in series with the feed pump, and a recirculating line having a bypass around the center wall;

FIG. 6 is a schematic view of a modification showing the recirculating pump between the center and water walls and including separate recirculating lines for the center and water walls; and

FIG. 7 is a temperature-enthalpy diagram at supercritical pressure.

In the vapor generator selected to explain this invention, the furnace walls surrounding and defining the com- 3,213,835 Patented Oct. 26, 1965 bustion space, and by custom referred to as waterwalls, are formed of a plurality 'of vertically arranged, parallel flow, closely spaced tubes, which may be welded to adjacent tubes to form a substantially gas tight combustion space. The tubes are connected together at the top and bottom by headers. The furnace is divided into two combustion chambers by a vertically extending centerwall comprising a panel of vertically extending parallel flow tubes joined at their tops and bottoms by headers. Because it is necessary to keep the tube sizes of the centerwall within practical limits and because the transverse extent of the centerwall in much less than the transverse extent of the waterwalls, the flow area of the centerwall is necessarily much less than the flow area of the waterwalls. The waterwalls may, for example, have a flow area three times that of the centerwall. Such a furnace is diagrammatically shown in FIG. 1, but for a more detailed showing and explanation, reference may be made to an application of Willburt W. Schroedter for Vapor Generator Organization Serial No. 127,396 filed July 27, 1961, now Patent No. 3,135,243 issued June 2, 1964, and assigned to the same assignee.

In the operation of vapor generators, custom and experience has dictated that minimum rate of circulation of the water or working medium in the heating tubes must be maintained at all times while there is fire in the furnace in order to prevent the burning of the tubes, forming the walls of the furnace, and containing the working medium, usually purified water. Custom and experience has indicated that an average velocity of about 3 feet per second, for example, is the minimum safe velocity at all operating conditions of the generator to provide the turbulent flow required to prevent localized heating and burning of the tubes. The centerwall and waterwalls are arranged in series with the working medium forced by the feed pump first through the centerwall and then through the waterwalls on its way to the superheater and the turbne where power is extracted from the working medium and the working medium returned to the feedwater pump. With the difference in flow area between the centerwall and waterwalls, indicated above, the centerwall will have a velocity of the working medium about three times the velocity of that medium at the entrance to the waterwalls. As the medium is heated and expands in the centerwall, it will expand and occupy a greater volume, so that the velocity of the medium at the outlet of the centerwall will be greater than the velocity of that medium at the entrance to the centerwall and for the same reason the velocity of the medium leaving the waterwalls will be much greater than the velocity 'of the medium entering the waterwalls. Although the velocity of the medium at the centerwall inlet is less than at the centerwall outlet, this lower velocity in the center Wall inlet is greater than the velocity of the medium in the waterwalls, due to the difference in flow area, so that the lowest velocity of the medium in the tubes will normally occur at the waterwall entrance and that is the portion of the through-flow system which is normally taken as the point at which the velocity should be maintained at 3 feet per second.

In order to avoid an excess of through-flow through the tubes at low load and still maintain the required 3 feed per second velocity with tubes of a practical diameter, of say 1 /2 inches, recirculation of fluid from the outlet of the waterwalls to the inlet of the centerwall and through the center and water wall in series may be provided and sufiicient of the medium is recirculated to provide the 3 feet per second velocity at low loads of 5 percent or even less. Such a recirculation system is shown and described in an application of Willburt W. Schroedter for Recirculating System for Steam Generator Serial No. 127,395 filed July 27, 1961, now Patent No. 3,135,252 issued June 2, 1964, and assigned to the same assignee, to which reference may be made for a more complete disclosure of the operation and advantages of the recirculating system in a once-through generator. The present invention utilizes many of the elements of the Schroedter recirculating system and is an improvement over that system.

In the present invention a bypass is provided by means of which only a selected portion of the total recirculating fluid is passed through the centerwall. With the circulation through the centerwall thus limited, adequate velocity is maintained in the centerwalls, at low loads, while the velocity at the waterwall entrance is maintained at the desired 3 feet per second. If desired a path in parallel with the path through the centerwall is provided around the center wall for a portion of the recirculating fluid so that only a portion of the recirculating fluid goes through the centerwall but all of the recirculating fluid will go through the waterwalls, or if desired a portion of the pump output may be used to recirculate water through the centerwall only and the remainder of the recirculating pump output used to recirculate medium through the waterwalls only. In each case, when the pressure drop through the centerwall, exceeds a predetermined amount, recirculation through the centerwall ceases and the only flow through the centerwall will then be the through-flow which will be at a velocity greater than the necessary 3 feet per second. The entire recirculating pump capacity will then be utilized to maintain a safe velocity of working medium through the waterwalls without having to overcome the pressure drop across the centerwall.

In the control of vapor generators and particularly the feedwater flow and firing rate thereof, it is desirable to obtain a temperature measurement which will give an accurate indication of heat input to the working medium or heat available to be picked up by the medium within the center and water walls. Such a signal should have a quick response and hence should be near the beginning of the heat absorbing system and for the sake of accuracy should be in a portion of the system where the change in specific heat is as near uniform as possible. It is, therefore, desirable to measure the temperature of the medium after it has been exposed to heating by the combustion gases and preferably before its temperature has been raised above the critical temperature in supercritical generators or the boiling point in subcritical generators where there is comparatively large change in specific heat for a small change in temperature. By placing the temperature indicator at the outlet of the centerwall and by reducing the quantity of hot recirculating fluid admitted to the entrance of the centerwall, a temperature reading can be obtained which will be rapid because it is in the very first part of heating system in the combustion zones and will be accurate because the comparatively cool feedwater will not be raised to as high a temperature with the smaller quantity of hot recirculating fluid as would be the case if all the hot recirculating fluid was combined with the feedwater and passed through the centerwall.

It will thus be seen that this invention provides means for obtaining a more accurate and rapid indication of the heat input to the Water tubes of the furnace and in addition increases the efliciency of the system by reducing the pressure drop across the centerwall over that occasioned when the entire recirculating fluid is passed through the centerwall.

An embodiment of this invention is shown in FIG. 1 which through-flow working fluid, usually chemically treated water, is taken from the water supply tank 42, by the feedwater pump 22, by way of pipe 44, and delivered under pressure via pipe 46, to the mixing chamber 20. The chamber has a discharge connection 200 to the header 24, connecting the bottom of the tubes forming the centerwall 12. The chamber 20 is normally located near the top of the vapor generator in order to obtain the benefit of the pressure differential between the cold water in line 206 and the heated fluid in the furnace walls. The tops of the tubes-12 are connected to a header 26, from which downcomers 28, connect to headers 30, connecting the bottom of the tubes 14, forming the waterwalls of the furnace 32. From the mixing chamber 20, the working fluid passes to the header 24 and then through panels of parallel heating tubes, formed of tubes 12 and 14, with the panels of tubes 12 connected in series with the panels of tubes 14 and forming radiant and/or convection heating surfaces exposed to the heat of the hot gases in the furnace. These tubes 12 and 14, form waterwall linings within the furnace chamber 32 as shown in more detail in the Schroedter application for Vapor Generator Organization, referred to above.

After passing through the tubes 12 and 14, the fluid passes to outlet header 18 and pipe 300, and thence in one direction to superheater inlet header 48, through superheater 50, outlet header 52, steam pipe 54, to a turbogenerator comprising turbine 56, driving an electric generator 58. The working fluid pases in another direction from header 18, through the recirculating pipe 60 to the recirculating pump 10. The steam from the turbine 56 is exhausted into condenser 62, the condensate passing through condenser pump 64, the feedwater heater 66, back into water reservoir 42, by way of pipe line 68, thus completing the steam power cycle.

The amount of working fluid recirculating through the water heating tubes 12 forming a centerwall in the furnace, and the tubes 14, forming the waterwalls in the furnace, is automatically regulated by the pump 10, driven by the constant speed electric motor 16. The pump takes fluid through line 60 from the header 18, connecting the tops of the tubes forming the waterwalls 14, and delivers it to the mixing chamber 20, which receives Water from the feed pump 22, also.

In operation the fuel supply and the feedwater supply of the steam generator may be controlled by any conventional control system to provide fuel and feedwater supply in accordance with load. In the embodiment chosen to illustrate the invention, the control system may be divided generally into two control systems. One system controls the turbine output and pressure by heat input. The other system controls feedwater flow and steam temperature in response to heat absorption. Both systems are intermeshed by a load output signal to the feedwater control and by a steam temperature signal to the heat input.

The one system is controlled generally by the load regulating computor 70, which provides error signals from desired load, pressure and frequency to regulate and maintain the proper required output over the full load range. Computor output signals are sent to the controls for feedwater 40, fuel and air 38, and to the turbine governor 72. The control action for fuel and air is accomplished by computer 70, so that fuel input through line 39, is regulated by valve 38 to maintain the proper steam generator output.

The other system coordinates the various systems controlling the internal steam generating process. Feedwater control is accomplished by a feedwater valve 40, in conjunction with a flow meter 74. Feedwater valve 40 is adjusted in accordance with heat input which in turn is adjusted by load. Heat input is sensed by centerwall temperature by a temperature responsive device 76, in the outlet from the centerwall. In addition the feedwater controller receives a signal from the combustion control system to maintain the proper fuel and feedwater relationships. The waterwall temperature is also adjusted to maintain a proper ratio of injection to the superheater for steam temperature control (not shown) to total feedwater flow.

The amount of working fluid recirculating through the tubes 12 and 14 is controlled entirely by the free running, uncontrolled pump, 10 floating on the one-through circuit and particularly, as shown in FIG. 1, the portion of the once-through circuit formed by tubes 12 and 14 and is located in the line 60 connecting the outlet header 18 with the mixing tank 20 and forming a part of the recirculating circuit connecting the outlet header 18 of the waterwall 14 with the inlet header 24, of the centerwall 12. A branch 78 from the line 60 in the outlet of the pump connects with a mixing chamber 80 in the line, or downcomer, 28 where the recirculating fluid from the waterwall outlet is mixed with the through-flow from the centerwall outlet and then conducted by the downcomer to the waterwall inlet 30. The centrifugal pump 10 is driven by a substantially constant speed mechanism shown as a constant speed electric motor 16 having an off and on switch 84 in the power lines 86, for the purpose of starting and stopping the recirculating pump 10. The conduit 60 is provided with a check valve 88 and the branch line or bypass 78 is provided with a check valve 82 and if desired the line 60 may be provided with a throttle valve 90 for gradually cutting in the recirculating flow when a start at full flow might produce too much of a thermal shock.

In explaining the self-regulating features of the recirculating pump it should first be noted that it is a centrifugal pump in which the head of the fluid pumped varies inversely with the quantity of the fluid being pumped. It should also be noted that the pump is pumping very hot fluid received from the header 18 at the top of the waterwalls and is delivering a portion of it to a mixing chamber Where it is mixed with the relatively cool feedwater to provide a cooler mixture which is fed to the centerwall where some heat is added and the temperature raised before the mixture is fed to the mixing chamber 80 and mixed with the remainder of the hot recirculating fluid from the header 18 and fed to the header 30 at the entrance to waterwalls. As explained at the beginning of this application custom and experience has dictated that the velocity of flow from the header 30 at the entrance to the waterwall 14 should not fall below the selected rate, of say 3 feet per second, for any normal operating condition of the steam generator. The 3 feet per second flow from the header 30 represents a fixed volume, that is, gallons per minute, regardless of temperature, as the flow area is substantially constant. However, as the temperature of the mixture leaving the header 30 and the temperature of the mixture leaving the header 18 and being pumped by the pump 10 as well as the temperature of the mixture leaving the header 24, at the entrance to the centerwall all diverge with increases in feedwater flow, it is apparent that the gallons pumped by the pump 10 will vary with respect to the gallons leaving the header 30 due to this temperature alone, in addition to the variation in discharge characteristics of the pump 10, due to the increase in head imposed by the increase in feedwater flow through the furnace walls.

In the Schroedter application structure for Recirculating System for Steam Generator, Serial No. 127,395 new Patent No. 3,135,252 issued June 2, 1964, referred to above, the temperature of the fluid leaving the water walls varied from about 730 at low loads, say 5 percent, to about 762 at 100 percent load. This is the temperature of the Water which is being recirculated and is substantially the same in the structure incorporating this invention. In the Schroedter application structure the temperature of the fluid leaving the centerwall which included all of the hot recirculating fluid varied from about 725 at low load where the fluid was a mixture of recirculating fluid and through-flow fluid to about 670 at 100 percent load where the mixture did not include any recirculating fluid and was composed entirely of through-flow feed water. In the Schroedter application structure also the temperature of the fluid at the centerwall inlet varied from about 720 at low loads, say 5 percent, down to 590-600 at 100 percent load. The high temperatures at the low 6 loads are occasioned by the incorporation of a large proportion of hot recirculating fluid with the small proportion of the cool feedwater through-flow with the cooler temperatures at the higher loads are occasioned by little or no hot recirculating fluid and a large proportion of, or entirely, cool feedwater flow.

In the present invention a portion of the hot recirculating fluid is bled off, or bypassed, through the bypass 78 to the entrance to the waterwall and it never reaches the mixing chamber 20 to be mixed With the cool feed- Water fed to the entrance to the centerwall. Hence the mixture fed to the centerwall entrance includes a much smaller proportion of the hot recirculating fluid and a larger proportion of the cool feedwater flow so that at low loads the temperature of the fluid entering the centerwall may be in the neighborhood of 680 and the temperature at the centerwall outlet may be in the neighborhood of 690 which is below the critical temperature and below the region in which a comparatively large change in specific heat takes place for a comparatively small change in temperature. This is shown by the line 91 in the temperature-enthalpy diagram of FIG. 6 which shows that in the region above 703 a large change in ethalpy produces a comparatively small change in temperature. This becomes of importance because it is desired to take a temperature reading T, FIG. 1, as a signal to be incorporated into the control system which will give an indication of the heat input to the fluid passing through the center and water wall heater tubes. By keeping the temperature below the critical temperature and observing the rise in temperature of the fluid passing through the centerwall it is possible to obtain a clear indication of the heat input to the fluid or the heat being absorbed by the fluid in passing through the centerwall without the distortion due to the different rate of enthalpy change on opposite sides of the critical temperature. By staying below a temperature of about 690 the ratio of temperature to enthalpy is substantially a straight line or nearly constant so that an accurate indication of heat absorbed may be obtained. By measuring the temperature rise in the center walls a quick response to any changes is obtained because the effect of the changes is observed at substantially the very beginning of the through-flow through the generator. Hence a much quicker response is obtained than would be the case if the temperature were observed at the outlet of the waterwalls.

The valve 82 in the bypass line 7 8 may be a combined throttle and check valve by means of which the restriction in the bypass can be adjusted so as to regulate the quantity of fluid that is bypassed and the quantity that is directed through the centerwall. By adjusting the valve 82 so that the minimum combined feedwater and recirculating flow through the entrance to the centerwall 12 is approximately the critical velocity of, say, 3 feet per second, a major portion of the hot recirculating fluid will bypass the centerwall and enter the waterwalls directly. The velocity of the fluid passing through the centerwall is thereby greatly reduced and consequently the pressure drop incident to that high velocity is also reduced, reducing the load on both the feedwater and the recirculating pumps. As the percentage of feedwater flow is increased, and the pressure drop across the centerwall increases, the recirculating flow through the centerwall will be reduced because the pressure drop across the centerwall will increase at a faster rate than the pressure drop through the throttle valve 82, so that the recirculating flow through the centerwall will be completely cut off while the recirculating flow is continued through the waterwalls. When the recirculating flow through the centerwall is stopped the necessary velocity through the centerwall is maintained by the feedwater through-flow alone. As indicated in the Schroedter recirculating application the recirculating pump has a characteristic of reduced volume with an increase in head pumped against, measured in feed of water pumped, so that the increas- 7 ing head against which the pump 10 must pump will gradually reduce the recirculating flow first cutting oif the recirculating flow through the centerwall and then cutting 01? recirculating flow through the waterwalls when the pressure drop in the waterwalls and the recirculating system equals the no-delivery pressure head of the pump '10.

The above description was of the structure shown in FIGS. 1 and 2 in which the pump 10 is located in the portion of the recirculating system path which conducts recirculating fluid only and hence the pump 10 pumps the hot recirculating fluid of 720-750 or 760 and pumps only that fluid.

A modification is shown in FIG. 3 in which the pump 10 has been moved from its position in the recirculating line 60 to a position in the line 300 connecting the waterwall and the superheater where it will pump a mixed flow including both the recirculating flow and the throughflow. It should be noted that in this modification, when the pressure drop through the waterwall and the recirculating path including the valve 82 equals the operating head for a given pump flow which may be described as the no recirculation delivery pressure of the pump, then recirculation through the waterwalls will cease but the through-flow will continue to pass through the pump 10 or through the bypass 104 and the check valve 106 around the pump whether the pump is idling or is stationary or is being driven as a turbine by the through-flow. Valves 100 and 102 on opposite sides of the pump may be used for isolating the pump or removing it for repair or replacement. This modification operates in substantially the same manner as that of FIG. 2 except that the pump pumps the mixed flow instead of only the recirculating flow. Recirculating flow is bypassed through the bypass 78 and the adjustable valve 82 bypassing the centerwall and limiting the recirculation flow through the centerwall. This provides the reduced pressure drop across the centerwall and the improved temperature conditions of the FIG. 2 structure.

FIG. 4 shows another modification in which the pump 10 is placed at the entrance to the centerwall and pumps the mixed flow including the recirculating and the throughflow. In this location, however, the pump pumps a much cooler liquid which as indicated in the Schroedter application has some advantages from the standpoint of pump size and operating conditions. Like the modification in FIG. 3, the pump may be isolated by the valves 100 and 102 and bypassed by the line 104 and the valve 106. The bypass 78 connects the centerwall entrance and the pump outlet with the mixing chamber 80 at the outlet of the centerwall 12. The valve 83 acting as an adjustable resistance controlling the quantity of fluid passed through the bypass 78 around the centerwall. This is mixed fluid which is bypassed and serves to reduce the velocity and the pressure drop in the centerwall. When the drop across the waterwalls produced by the throughflow equals the pump operating head for the pump flow (the no recirculation flow delivery pressure of the'pump 10), recirculation through the waterwalls will stop. A portion of the through-flow will continue to pass through the bypass 78 giving a reduced velocity and pressure drop in the center wall. Valve 83 may be closed if it is desired to direct all of the through-flow through the centerwall. In this modification the advantage of the reduced temperature at the centerwall outlet is not obtained because all of the hot recirculating fluid is mixed in mixing chamber 20 with the feedwater flow thus increasing the temperature to the centerwall inlet and at the centerwall outlet over that of the two previous modifications.

FIG. 5 shows another modification with the recirculating pump moved up to a position between the center wall and the waterwalls where it will pump the fluid at a temperature intermediate that at the centerwall entrance and that at the waterwall outlet. As in the modifications of FIGS. 3 and 4 the pump may be isolated by the valves 100 and 102 and bypassed by the line 104 and the valve 106. This modification operates in a manner very similar to that of FIGS. 2 and 3 in that only a portion of the recirculating fluid is mixed with the feedwater in the mixing chamber 20 to pass through the centerwall thus providing a comparatively cool mixture for the centerwall entrance. Because a majority of the recirculating flow passes through throttling check valve 82 and bypass 78 the pressure drop across the centerwall is reduced at low loads. The throttling portion of valve 82 permits adjustment of the bypass flow and the check valve portion prevents bypassing of the waterwalls through line 60 at high loads. Because of the low temperature entering the centerwall the temperature of the fluid leaving the centerwall is in the preferred low temperature region giving the more accurate temperature signal for control purposes. As in the modification of FIGS. 2 and 3 the centerwall recirculation is first automatically cut 011? by the increase in pressure drop across the centerwall incident to an increase in through-flow then the recirculation through the waterwalls is automatically cut off by the increase in pressure drop incident to the increase in through-flow through the waterwalls. As in the other cases where the pump 10 pumps the mixed recirculating and through-flow the through-flow may pass through the pump or through the bypass 104 when recirculation has stopped.

FIG. 6 shows a still further modification in which the pump 10 is placed between the centerwall and the waterwall and may be isolated and bypassed in the same manner as the FIG. 5 modification. The recirculating path around the waterwalls includes the line 61 and the check valve 89 and carries only the recirculating fluid from the waterwall which empties into mixing chamber 80 at the outlet of the centerwall, the check valve 89 preventing return flow from the centerwall outlet to the waterwall outlet in the same manner that adjustable check valve 82 does in the FIGS. 2, 3 and 5 modifications. In the FIG. 6 modification the recirculation fluid for the centerwall is taken from the inlet to the waterwalls instead of directly from the outlet from the waterwalls as in the previous modifications. This provides cooler water to be supplied to the mixing chamber 20 to be'mixed with the comparatively cool water from the feed pump to be supplied to the centerwall. The bypass 79 from the inlet to the waterwall to the inlet to the centerwall includes the adjustable check valve 85 by which the quantity of fluid to be recirculated through the centerwall may be adjusted. A temperature below the critical temperature at the centerwall outlet is thus assured over substantially the entire operating range. Assuming that the waterwall has about three times the flow area of the centerwall and that therefore the pressure drop through the centerwall for the same flow will be about three times that of the waterwall and assuming that the recirculating pump has an operating pressure which will provide no recirculating delivery at about 60 percent of the load, or the throughfiow; then if the adjustable valve 85 is adjusted so that the flow through the centerwall is at some selected critical velocity say 3 feet per second, at a low selected load percentage say 5 percent then as the load and the throughfiow increases the pressure drop across the centerwall would increase much more rapidly than across the waterwall and the pressure in the mixing chamber 20 and bypass 79 would increase so that at approximately 20 percent load the pressure drop across the centerwall would equal the operating pressure of the pump and the recirculation through the centerwall would stop automatically. The through-flow would then maintain the required safe velocity through the centerwall. v The recirculation through the waterwalls, however, would continue until the pressure drop across the waterwalls equalled the operating pressure of the pump which would not occur until about 60 or more percent of the total load or throughflow was obtained at which point the recirculation 9 through the Waterwalls would automatically stop and then only the through-flow would go through the center walls, the pump or the bypass 104, and the waterwalls to the superheater.

In each of the modifications the centerwall has a smaller flow area than the waterwalls and hence requires less recirculation to maintain the critical velocity through the centerwall tubes. In each case only a single pump is used for recirculating fluid and only a portion of the pump fluid is recirculated through the centerwall, thus reducing the pressure drop across the centerwall and saving pump power. In the modifications of FIGS. 2, 3, and 6 because of the introduction of a smaller amount of hot recirculating fluid to the through-flow entering the centerwall a smaller temperature rise is experienced at the outlet of the centerwall and this is particularly true in FIG. 6 where even the recirculating fluid that is mixed with the feedwater through-flow entering the centerwall is cooler than that taken from the waterwall outlets for recirculation through the waterwalls. Further recirculating pump power is saved by not having to pump against the pressure drop through the centerwall and having to pump only against the pressure drop through the waterwalls at the higher powers after the recirculation through the centerwall has stopped.

It is to be understood that the invention is not limited to the specific embodiments herein illustrated and described but may be used in other ways Without departure from its spirit and that various changes can be made which would come within the scope of the invention which is limited only by the appended claims.

What is claimed is:

1. A forced flow once through supercritical vapor generator having a variable vapor demand rate and a through flow line including separate sections connected in series for series through flow and requiring a minimum safe velocity of working fluid therethrough, one section having a smaller flow area and a higher through flow fluid velocity than the other, means connected with the inlet of said sections supplying through flow fluid at supercritical pressure to said inlet, a pump connected across both sections and recirculating working fluid through both sections and having an inlet and an outlet connected with said through-flow line and combining said recirculating fluid with said through flow fluid and means including said pump responsive to pressure diflerences between the outlet of the second section and the entrance to each section decreasing recirculating fluid through each section With increase in through flow, and means directing a portion of said recirculating fluid through said second section only and bypassing said one section to restrict the velocity of combined fluid through and the pressure drop across said one section.

2. A forced flow once through vapor generator having a variable vapor demand and a through flow line including first and second fluid heating sections connected in series for series through flow and requiring a minimum safe velocity of fluid flow therethrough, means connected with an upstream portion of said first section for supplying through flow working fluid for both sections in accordance with demand, said first section having a smaller through flow area and a higher through flow velocity than said second section, common recirculating pump means having a limited pressure and having an outlet connected with the inlets of both sections and having an inlet connected with the outlet of the second section and having a delivery pressure and volume at least sufficient to bring the velocity of Working fluid in said sections to a selected safe velocity when the through flow alone is insufiicient to do so, said pump means outlet including a common passageway connected with a separate recirculating fluid passageway for each section, one separate passageway connected with the inlet of the first sec tion, the other separate passageway connected with the inlet of the second section, said common and said separate passageways having generally constant recirculating fluid flow areas during all recirculating flow through the respective sections, said pump means and said separate passageways supplying separate streams of recirculating fluid simultaneously to said sections at a pressure greater than the pressure at the entrance to said sections due to through flow alone when the through flow alone does not provide said safe velocity, increasing through flow for increased vapor demand creating an increased pressure at the inlet to said first section greater than the pressure of said recirculating fluid supply to said first section, while creating a smaller increase in pressure at the inlet to said second section to automatically discontinue recirculation through said first section while continuing recirculation through said second section.

3. A forced flow once through supercritical vapor generator having first and second heating sections each having an inlet and an outlet and arranged in series for series flow of the working fluid, the first section having a smaller working fluid flow area than the second, a common p ump having an inlet and an outlet and pumping recirculating fluid for both sections, means connecting said pump inlet with the outlets of both series connected sections, and the pump outlet with the inlets of both said sections, said means connecting said pump inlet with the outlet of said first section, and said means connecting said pump outlet with the inlet of said second section comprising, With said pump, means connecting said sections in series for series flow through said sections the connections to said second section directing at least a portion of the pumped recirculating fluid through said second section only and the connections to said first second simultaneously directing the remainder of the pumped recirculating fluid through said first section only.

4. A forced flow once through vapor generator having means for supplying hot gases and having a variable vapor demand and having a through flow line including series connected first and second working fluid heating sections, each section having an inlet and an outlet and exposed to said hot gases and means connecting the outlet of the first section with the inlet of the second section, each section requiring substantially the same minimum safe velocity of working fluid, means connected to the inlet of said first section for supplying a through flow of working fluid for both sections in accordance with demand, said first section having a working fluid flow area smaller than said second section and having a higher through flow velocity than said second section at all times, recirculating means for recirculating fluid through both sections, said recirculating means including pump means for recirculating fluid through both sections to supplement said through flow and bring the working fluid velocity in said sections to said minimum velocity and means continuously open for recirculating fluid flow and having a generally constant fluid flow area continuously connecting said pump means when delivering recirculating fluid with both the inlet and the outlet of said second section for continuously directing at least a portion of the recirculating fluid pumped by said pump means through said second section only and means directing the remainder of the recirculating fluid pumped by said pump means to the inlet of said first section and means combining said recirculating fluid with said through flow fluid, said pump means constructed and arranged to reduce the quantity of recirculating fluid as said through flow increases.

5. A generator as claimed in claim 4 in which said recirculating means includes means having a generally constant recirculating fluid flow area connecting both the inlet and the outlet of said first section with said pump means to thereby connect both the first and second sections in parallel across said pump means.

6. A forced flow once through vapor generator having a variable vapor demand range and a through flow line including first and second working fluid heating sections, each section having an inlet and an outlet and composed of a plurality of tubes requiring substantially the same minimum safe velocity of working fluid in the tubes and means connecting the outlet of the first section with the inlet of the second section for flow of through flow 'WOIking fluid through said sections in series, said first section having a flow area substantially smaller than said second section and having a higher through flow velocity than said second section at all times, means connected to the inlet of said first section for supplying a through flow of working fluid for both sections in accordance with demand which through flow in a selected portion of the range of demands is insuflicient to produce said safe velocity in either section, recirculating means for recirculating working fluid through said sections, said recirculating means including common pump means for recirculating working fluid through both sections to supplement said through flow and bring the working fluid velocity in said sections to said minimum velocity in said selected range, and means including separate passageways having a portion in common and having generally constant recirculating fluid flow area during recirculation flow connecting said pump in fluid flow relation across said sections and with said through flow line, said last mentioned connecting means connecting the outlet of said second section with the inlet of said pump and directing a separate stream of recirculating fluid to the inlet of each respective section, said pump having and applying across both sections through said passageways a pressure boost greater than the pressure drop through said sections due to through flow alone in said selected range but having a pressure boost applied across said first section less than said pressure drop through said first section in the remainder of the range of demands, said through flow at increasing demands increasing the pressure drop across said sections inversely as the flow area to first discontinue recirculation in said first section while continuing recirculation in said second section.

7. In a forced flow once through vapor generator having a variable vapor demand rate range and a corresponding range of rate of through flow and a through flow line and means supplying a through flow of working fluid to said line, the combination of separate through flow sections including a downstream section and an upstream section connected in series in said line for flow of working fluid therethrough, each section having an inlet and an outlet, one section having a smaller through flow area than the other, the pressure differential across each section, due to through flow alone varying with the rate of through flow through the respective section, conduit means for recirculating fluid having 'a generally constant fluid flow area during flow of recirculating fluid connecting the outlet of the downstream section with the inlet of 'both sections, pump means having an outlet and an inlet in said conduit means forcing recirculating fluid through said sections in addition to the through flow, said conduit means including separate conduit means having generally constant fluid flow areas during recirculating fluid flow connecting the pump outlet with the respective section inlet said pump means having a limited head applied across said sections which is greater than said pressure differential across said one section through a selected portion of the range of rate of through flow and less than said pressure differential across said one section throughout the remainder of the range of the rate of the through-flow, said limited head applied across said other section being greater than the pressure diflerential across said other section throughout a range including and greater than said selected portion.

8. A combination as claimed in claim 7 in which said conduit means includes separate conduit means having generally constant fluid flow areas during recirculation flow connecting the pump means inlet with the upstream section outlet.

9. A forced flow once through supercritical steam generator having a through flow line for working fluid including first and second heating sections connected in series through flow arrangement and having means for heating the working fluid in said sections including heating said fluid to above critical temperature in said second section, means supply through flow Working fluid to the inlet of said first section, recirculating pump means having an inlet and an outlet forming part of said through flow line for pumping through flow working fluid and recirculating working fluid at supercritical temperature from said second section including means having a constant recirculating fluid flow area connecting said outlet of said second section with said inlet of said pump means and the outlet of said pump means with said inlet of said second section including means mixing the supercritical recirculating fluid with said through flow working fluid, said recirculating pump means including means having a constant recirculating fluid flow area connecting the outlet of said pump means with the inlet of said first section and including means providing the recirculating fluid from said pump means into two separate streams, one stream flowing through said means connecting with the inlet of said first section and the other stream flowing through said means connecting with the inlet of said second section and circulating a smaller quantity of fluid in said one stream through said first section than in said other stream through said second section.

10. A generator as claimed in claim 9 in which said recirculating pump means and its connecting means includes means having a constant recirculating fluid flow area connecting the outlet of the first section with the outlet of the second section and the inlet of said pump whereby all of the recirculating fluid is mixed with the through flow between said first and second sections and a portion of said mixed flow including said smaller quantity of fluid is mixed with the through flow entering said first section.

11. A generator as claimed in claim 10 in which the through flow supplied to said first section is comparatively cool and the recirculation flow supplied to said first section is at less than critical temperature and said heating means heats said combined through flow and recirculating flow to less than critical temperature in said first section and provides a substantial linear temperature heat input relation in said fluid in said first section, temperature responsive means in the outlet of said first section indicating the heat input to said fluid supplied in said first section.

12. A method of operating a forced flow once through vapor generator having two heating sections in series through flow arrangement with the first section having a smaller through flow area than the second section and a common recirculating fluid pressure source having a limited boost pressure which comprises the steps of forcing a through flow of working fluid through said first section and then through said second section in series, recirculating working fluid from said source in separate streams to and through said sections in addition to said through flow, increasing the through flow through both sections and, due to the difference in flow areas of the sections, thereby raising the pressure drop across said first section more than the pressure drop across said second section and to a pressure difference greater than said boost pressure to thereby block one stream and prevent further recirculation through said first section while continuing recirculation through said second section.

'13. In combination with a once through vapor generator having a variable vapor demand rate and a through flow line including separate sections connected in series for series through flow, each section having a pressure drop across the respective section varying with fluid flow through the section, and requiring a predetermined safe velocity flow of working fluid, one section having a materially smaller fluid flow area than another, means supplying a through flow of working fluid to said sections, a recirculating pump having a limited pumping head and connected in fluid flow relation across said sections by separate passageways, one for each section, and having a portion in common, said passageways having a generally constant flow area during flow of recirculating fluid and having means blocking reverse flow in at least one of said passageways, said pump having -a pumping head at each section balanced by the pressure drop through each respective section and the pressure loss in the respective constant flow area passageways and a decreasing pumped volume with increasing pumped head to decrease recirculation flow with increased through flow, said head and volume being suflicient at all times to provide with said through flow at least said safe velocity in said sections, said pressure drop increasing with increased through flow more in said one section than said other section and automatically blocking recirculating flow through said first section by exceeding the pumping head across said section while continuing recirculation flow through said other section and through flow through both sections.

14. A vapor generator as claimed in claim 13 in which the pump is located in a line parallel with the throughflow line.

15. A vapor generator as claimed in claim 13 in which the pump is located in the through-flow line at the outlet of the downstream section.

References Cited by the Examiner UNITED STATES PATENTS 2,324,513 7/43 Junkins 122-451 3,03 8,453 7/ 62 Armacost 122406 FOREIGN PATENTS 451,992 8/36 Great Britain. 506,388 5/39 Great Britain. 768,201 2/57 Great Britain.

PERCY L. PATRICK, Primary Examiner.

JAMES W. WESTHAVER, FREDERICK L.

MATTESON, JR., Examiners. 

1. A FORCED FLOW ONECE THROUGH SUPERCRITICAL VAPOR GENERATOR HAVING A VARIABLE VAPOR DEMAND RATE AND A THROUGH FLOW LINE INCLUDING SEPARATE SECTIKONS CONNECTED IN SERIES FOR SERIES THROUGH FLOW AND REQUIRING A MINIMUM SAFE VELOCITY OF WORKING FLUID THERETHROUGH, ONE SECITON HAVING A SMALLER FLOW AREA AND A HIGHER THROUGH FLOW FLUID VELOCITY THAN THE OTHER, MEANS CONNECTED WITH THE INLET OF SAID SECTIONS SUPPLYING THROUGH FLOW FLUID AT SUPERCRITICAL PRESSURE TO SAID INLET, A PUMP CONNECTED ACROSS BOTH SECTIONS AND RECIRCULATING WORKING FLUID THROUGH BOTH SECTIONS AND HAVING AN INLET AND AN OUTLET CONNECTED WITH SAID THROUGH-FLOW LINE AND COMBINING SAID RECIRCULATING FLUID WITH SAID THROUGH FLOW FLUID AND MEANS INCLUDING SAID PUMP RESPONSIVE TO PRESSURE DIFFERENCES BETWEEN THE OUTLET OF THE SECOND SECTION AND THE ENTRANCE TO EACH SECTION DECREASING RECIRCULATING FLUID THROUGH EACH SECTION WITH INCREASE IN THROUGH FLOW, AND MEANS DIRECTING A PORTION OF SAID RECIRCULATING FLUID THROUGH SAID SECOND SECTION ONLY AND BYPASSING SAID ONE SECTION TO RESTRICT THE VELOCITY OF COMBINED FLUID THROUGH AND THE PRESSURE DROP ACROSS SAID ONE SECTION. 